Double case pump



April l7, 1945. A. J. NELSON DOUBLE CASE PUMP 2 Sheets-Sheet 1 Filed Dec-. 5; 1942 R //////w M 2 RQ J 2 IFQ g w m w m IQMQM W17!!! ARTHUR d. NELSON INVE Afro NE) April 17, 1945. A NELsdN I 2,374,122

DOUBLE CASE P iled Dec. 5, 1942 2 Sheets-Sheet 2 ARTHUR J. Ms M INVENT ZTTORNEY ends of the annular space.

Patented Apr. 17, 1945 some: as: 1

Arthur J. Nelson, San Francisco, Calif.

Application December 5, 1942; Serial hlo.

14 Claims. (crios iotlfi I This invention relates to 'high' pressurecentrifugal pumps, and more particularly to multistage pumps of the type employing double casings. :High pressure centrifugal pumps usually consist of a plurality of impeller elements'spaced y tf turi e z-o i t Mim -i nt nter axially on the drive shaft, and a casingstructure defining working passages for the several impeller elements. In order'that the'casing portion defining the several volutes maybe assembled about the impeller elements, it is split axially intotwo halves. These halves of the casing are -formed of relatively complicated castings and,- in

addition to being split lengthwise, the castings usually are relatively thin walled so that the bas ing is inherently weak as regards intema1'pres-.,

sures. An accepted practice is toenclose the casing defining the several volutes within a second outer'casing, the .two casings'being spaced circumferentially, and to introduce .into the space therebetween a fluid pressure from the pressure side of the runners orimpellers. Thus a'force is exerted against the outer surface of theinner casing'to counteract the" internal pressures of the pump.

It is diflicult to efiect a positive'se'al between the twocasings at the ends of the 'pump for sealing the annular space therebetween' againstleak age of the high pressure fluid. This 'difflculty 'is amplified where pumps of this type 'are'used for pumping oil at high temperatures and delivering it under high pressures. Due to variations in temperature between the'inner and'outer casings, provision must be made in the seaiing arra'nge' 'ment for permitting relative movement or-tm casings. The importance of a positive seal forthe annular'space between'thecasings will be better appreciated if it be understood thatth'e temperature of the oil being pumped may be above its flash point, and hence leakage ofoil to atmosphere in the event of failure of the seal endangering both life and property;

It is, therefore, an objectof the present invention to provide a new and improved highpressure would eauseit to burst immediately into flame,

centrifugal pump of the double casing type which -i.

is mechanically strong, safe in operation, and particularly suitable for pumping inflammableliquids at high temperatures. Y

' Another object of the invention "is toprovid an improved high pressure centrifugal pump comprising an inner and an outer casing withanannular space therebetween communicating: with the discharge side of the pump, and havingnew and novel sealing-arrangements at the opposite vider a newrandimprovedisealing arrangement for the annular'space between casings ofa double casingc'high pressure centrifugalpump whereby direct=leakage of high-pressure iiuid from such spaceto-atmosphere-is precluded.- --';:A: further obiect-of-.;the invention is to provide arnew and improved arrangement for mounting .the-= inner casing of a high pressu're centrifugal pump within? asurrounding outer casing which includes means-for permitting relative movement of thecasingsand-at the same time precludes leakage of high pressure fluid from the space between-the casingsto atmospheres 1 Another object r of. the; invention. is to provide a new, andimprovedhigh'pressurecetrifugal pump of the double casing type and having a space between .acasings communicating with the pump idischargatetheicasingsbeingso designed as to require ia-Jninimum of machining operations and yeti-which may.be:fltted together into a compact assembly oi' extremely :higlr mechanical strength andinafluid tight relation. 1 i

----An ther object-of thepresent invention is to provide. a new-vand r improved impeller assembly ands-surroundingcasing therefor which minimizes-leakage of; high pressure fluid between adjaceiit'pllmp stagesiand which arrangement is of a 'fgreatly"simplifie'd'construction; v

In accordance with one form of the invention,

the'pump consistsessentially of *an impeller. unit, an "innericasingdefining; the working passages fertile-impellers; and an outer casing The outer casing is of a: relatively rugged cylindrical design,

having an-openend into'wh-ich may-be inserted in-the corresponding end wall of the outer casing. I

A fluid inlet {passage is; defined by openings pro- 'videdthroughthe' casings adjacent the openrend 'bf 'the casingrand-a fluid-outlet passage is defined by'a -second set: of; openings through the casings at a point adjacent the discharge stage.

An annular space betweenthe casingsis in conimunication with the discharge, passage so that the exterior'of thelrinner casing-is subjectto substantially discharge pressurertocounteract the outward forces of--the-' internal pressure.

The annular space :between the casings is sealed at -.a point-between the fluid inlet and outlet passages and adiacent'the -open endnof the outer casing by sealing means arranged between annular shoulders provided in the adjacent walls of the The sealing means at opposite ends of the annular space is so arranged that'in-the event of leakage of high pressure fluid past the sealing means at either end Of the annular space, such leakage will occur into low pressure fluid chambers and b recirculated within the pump. 1

Further objects and advantages of the invention will become apparent from a'perusal of the tional views of Figures 2 and 3, extending lengthwise through the inner casing to the second impeller stage at the opposite end of the .pump. By thus arranging the two low pressure stages at opposite ends of the pump, the stuffing box 4! at the suction end of the pump is subjected only to suction pressure, and the stufing box 59 at the opposite end of the pump is sub jected only to suction pressure plus one stage pressure; From stage i5; fluid hows successively through the adjacent stages i8, i1, i8 and Hi, from which last stage the fluid flows through i a. second pair of diametrically opposed passages ,33 and 34 through the inner casing Ii to the suction chamber for the next impeller stage 30. From the stage the fluid flows successively through the next adjacent stages 2|, 22

and 23, the last mentioned stage being the disfollowing description, taken in connection with the accompanying drawings, while the features of novelty characterizing the invention will be pointed out with greater-particularity in the appended clairns. I

In the drawings, Figure i is a dongitudinal sectional view through'a pump ccn'structed in accordance with one form ofythe invention; Figure 2 ice radial cross sectional view through the pump. taken along the line 7 -2 of Elgure 1;'Figure 3 is a radial cross sectional view, taken through the pump along" the lined-40f Figure I: Figure 4 is an enlarged longitudinal sectional view of an intermediate portlonof-the pump rotated through an angle of "90 degrees with respect to the plane of the sectlonal view of Figure 1 Figures 5 and 6 are'e'nlarged fragmentary views of details of the pump structure shown in I The pump as shown consists essentially of'an impeller unit comprising a; shaft l0 having a plurality of axially spaced minimal-imperler elements arranged thereon, an inner casing ll defining working passages for the impeller unit, and an outer casing I-Z'encI'os'i-ng the whole but spaced circumferentially from the inner casing. The pump in this instanceemploys ten stages, but it will become obvious as the-description proceeds that the features of the invention are independent of the exact number of stages employed which may be greater or-lessthanthe number shown.

The various impeller stages are indicated by 'the reference characters H to 23, inclusive, in the order of their relative positions in'the fluid circuit through the pump As illustrated more clearly in the enlarged view oi Figure 4, the impeller elements are each comprised of a disc portion 26 and a hub portion?! keyed to the shaft III. A snap ring 28 isprovided' in'an annular groove about theshaft and extends into an annular groove provided inthe hub portion 21 adjacent the inlet or suction edge ofthe'element for the purposeoi' preventing movement of the impeller element longitudinally "of the shaft in the direction of the nuid 'approach due charge stage of the pump. This staggered arrangement is for the purpose of eliminating high .pressure 'difl'erentlals between adjacent stages and for balancinghydraulic thrust." At the suction end of the pump a five stage differential exists between the first stage 14 and the seventh stage 20-. At the center of the pump only a four-stage difier'ential exists between the sixth stage l9 and the tenth stage 23; Elsewhere in the pump one-stage differentials appear across the suction wearing rings. This arrangement permits a material reduction in the length of the throttles. Openings 35 and 3B the inner and outer casings adjacent the tenth stage define afluid outlet passage, which passage is in'communication with the annular space Blhetween the two casings. a The-inner casing; defining the workingpassagesfor the rotor assembly, is formed by a pair of matching castings which may be assembled about the rotor and secured together" in any suitable manner as by bolts (not shown). CEt will be-obvious that castings of sucha complicated nature are inherently relatively: weak structural elements and incapable of withstanding any great internal pressures. In order to provide sufficient mechanical trength for the structure, the space 31 is filled with fluid under discharge pressure and the two inner casing halves are forced tightly together, the pressures obtaining internally thereof belngefiectively neutralized --by the pressure obtaining between the casings.

Theouter casing i2 is of a relatively-simple cylindrical design, the sidewall; of whiche-is sumciently thick to withstand the pressure developed by the pump. The outer casing is-open at the suction end, through which end the in.- ner casing assembly may be introduced. The

inner casing i provided with an-extension n defining the inlet passage 30 and bolting flange portion 39 by which the inner casing is fastened to the cover 40 for the open end of theouter casing. The shaft 10 of the impeller-unit extends through the stuihng box 4i mounted within an axial opening in the cover '40. The

in space. 31.

'ill'prevents rotation of thei'nner casing-within the outer casing, eliminatingthe necessity'of a key" therebetween, and also provides means 1 whereby the inner casing may readily be withdrawn from the outer casing, as with jack screws: The fluid inletpassage 29, 30 is sealed from the annular space 31 between the casings by an annular gasket 41 arranged between a pair of annular shoulders 45 and 46 provided on the adjacent surfaces of the inner and outer casings, respectively. The shoulders 45 and 46 are provided onthe respective casings near the open end of the outer casing on the opposite side of the inlet passage with respect thereto. Both gaskets 44 and 41 are of a compressible material; the latter being somewhat the thicker so as to insure a tight seal between shoulders 45 and 46 capable of withstanding the high fluid pressure obtaining seen failure of the seal between shoulders 45 and 4B, leakage of high pressure fluid therepast will take place into the low pressure fluid inlet chamber from whence it will be recirculated within the pump. It will be observed-that the inner casing extension 38, on'which the flange portions 39 are formed, extends axially from the peripheral edge of the inner casing so that a However,in the event of unforecompressiv force in a straight line is imposed bythe cover against the inner casing to'hoid the shoulders 45 and 46 tightly together. Moreover, the inner casing extension 36 bearsagainst the cover closely adjacent the inner circumference of the outer casing so that the shortest possible distance exists between the bearing-surface and thebolts 43. Thus the lever arm therebetween is short, the bending moment correspondinglyre. duced, and the likelihood of the seal between shoulders 45 and 46 becoming broken due to fatigue bending of the cover is minimized. This will be more readily understood when it isconsidered that if the cover werernade to'bear against the inner casing closely about the shaft, then the cover would tend to dis'h'outwardly in time under a heavy stress to permit separation of the shoulders. By the construction shown the inlet end of the inner casing is rigidly clamped between the outer casing cover and the,outer casing shoulder 46. t

By. virtue of the fact that the shoulder 46of the outer casing is disposed near the open end, it is comparatively easily accessible for accurately machining the surface so as to insure a good fit with the shoulder 45 on the inner casing."

The opposite end of the inner casing is provided with a cylindrical extension 5| which is 'adaptedto be received within an opening 52 provided-in the corresponding end wall 53 of the outer casing. In order to facilitate the assembly of the pump, the inner edge of the opening is slightly chamfered as indicated at 54 to guide the cylindrical extension into the opening '52. j I

'A sealin arrangement is provided between the inner and outer casings and is arranged within an enlarged outer end portion of the opening 52 in the outer casing end wall 53. The sealing means comprises an annular compressible member 56 of a suitable material, such as asbestos, and which is compressed into a substantially fluid tight engagement against the outer cylindrical surface of the inner casing extension 5|v by -means of an annular metal compression ring '51. The compression ring 51 is forced against the seal member 56 by the flange wall 58,0f the stufling'box assembly 59 provided for the correcasings.

spending end of-the shaft I6. The-stufling box assembly 59, fastened securely to the adjacent end wall 53' of the outer casing by means of a plurality-of annularly arranged stud bolts 6!, is provided with an annular shoulder 60-on the inner surface of thefiange which bears against the compression'ring' 51 through an annular gasket; 62. It will be observedthajt a space 63'exists between the'extremity of the inner casing 'extension 5i and the adjacent wall of the stuifing box 'flang'e 58 so that there 'is no obstruction to relative axial movement between the inner and outer casings at this end. The compression ring 51 hearing against theannular'seal member 56 extends beyond the end of the'inner casing extension 5| so that the extension doesfn'ot bear against the edge of the gasket 62, which may the cylindrical extension 5| engages with the circumference of the stuffing box 59, over'a .relatively small area only andwith a'relatively loose fit,'so that the space 63 is in substantial communication with the suction chamber 64 of the adjacent low pressureimpeller stage l5," Thus, in the event of a partial failure f the an'nularseal member 56, permitting leakage therepast from the annular space 31 between the inner and outer casings, the high pressure fluid will flow through the space 63 and back into the suction chamber 64, thegasket 62 precludingleakage of high pressure fluid from space 63 to atmosphere.

In order to prevent the fluid bein discharged from the various impeller stages from imparting 'a transverse thrust tofthe inner casing due to the tangentialflow of the liquid from the volute of .theworking .passage and thereby distorting theoppositeend seals, the volutes are so designed the suction chamber for thenext succeeding impeller stage. a y

For the last, or discharge stage 23, the inner casing defines a-double volute as is shown more clearly in the cross sectional view of Figure 2.

As in the case of the other stages;fluid emerging from the impeller disc is led; from the impeller, chamber atdiametrically opposed points through passages 65and 66. The discharge end 35 of the passage 65 is disposed in alignment with the discharge opening 36 of the outer casing l2, while the outer end of the passage 66 communicates with the annular space 31 between the come necessary so to do in the course of main tenance operations. Another importantadvam tage of this feature of the inventionis that the Thus a positive flow of liquid 'under hot oil in the annular space. 3.!- maintains av uniform temperature around the inner casing.

It will be observed that the impeller elements of the stages I4, 20-, 2|, 22 and 23 have their suction ends all facing in the same direction, toward the left, while the impeller elements of the remainingstages, namely l5 to lihinclusive, all face in theopposite direction, or toward the right. Thus, only inlet fluid pressure is imposed against the cover 40 and the. shaft packing of stufiing, box 4|, and fluid under only suction pressure plus pressure of one pump stage is imposed against the shaft packing. of stumng box 59 at the opposite end of the pump. By such an arrangement of the impeller elements on the shaft the resultant force in the axial direction. of the fluid flowing through I the. pump creates a minimum of thrust onopposite sides of the center of the pump.

In the assembly of the impeller unit, the various discs and bushing rings cooperating with the hubs of each may he slid over the end of the shaft and keyed in position. It will be observed, however, that the impeller discs of discharge stage 23 and the adjacent stage I!) are connected together in a back to back relation. Referring more particularly to the enlarged view of Figure 4, the hub 7 portions H and 12 of impellers of stages 23 and 19,

- outwardly extending annular rib 15 which is received within a cooperating annulargroove provided in the wall 16 of the two casing halves, When assembled about the rotor unit the casing halves engage the collar 14 with sufficient friction to hold it stationary with respect to the rotor; A slight space 1'! is left between the hub portions H and 12 to permit relative angular adjustment between the two impeller elements for aligning them with their respective keyways in the shaft. Since the resultant forces acting against the two impellers thus tied together is toward the right, only a single snap ring 18 need be provided at the right edge of the hub' .of the impeller element is to hold the pair of elements in the predetermined axial position. In order more efiectively to prevent flow of high pressure fluid from stage 23 to stage [9, the inner surface of the collar 14 may be provided with a plurality of grooves 8| which operate in the manner of a well known labyrinth type of packing. I

Having described the principles of the invention in what is considered to be an operative embodiment thereof, it is to be understood that various details shown are merely illustrative, and that the invention may be carried out by other means.

What I claim as new and desire to protect by Letters Patent is:

1. A pump comprising a multistage impeller unit, an inner casing defining the working passages for the severalimpellers of said impeller unit, an outer casing, a fluid inlet through said casings adjacent one end thereof, said outer casing having an opening'at the suction end thereof for assembly and disassembly of the inner casing relative to the outer casing, a fluid outlet through said casingson the opposite side of said inlet with respect tosaid open end, annular shoulders on said outer and inner casings adjacent said open end betweensaid inlet and said outlet and means for securing saidinner casing relative to said outer casing and for forcing said shoulders together into a seating relation. p

2. A pump comprising a-multistageimpeller unit, an inner casing defining the workingapassages for the several impellers of said impeller unit, an outer casing, a fluid inlet through said casings adjacent one end thereof, said outer casing having an opening at the suction end thereof for assembly and disassembly of the inner casing relative to the outer casing, a fluid outlet through said casings on the opposite side of said inlet with respect to said open end, an inwardly extending annular shoulder provided on the inner circumference of said outer casing between said inlet and said outlet, an annular shoulder on the outer circumference of said. inner casing-adapted to be seated against said first shoulder, and means for securing said inner casing relative to said outer casing and for forcing said shoulders into seating engagement.

3. A pump comprising a multistageimpeller unit, an inner casing defining the working passages for the several impellers of said impeller unit, an outer casing, a fluid inlet through said casings adjacent one end thereof, said outer casing having an opening at the suction end thereof for assembly and disassembly of the inner casing relative to the outer casing, a fluid outlet through said casings on the opposite sideof said inlet with respect to said open end, a removable cover for said open end, of the outer casing, said cover being subjected only 'to suction pressure obtaining in said inlet passage,fsaid inner. casing being. rigidly secured to said cover for preventing arelat'i've rotation of said casings, said cover providingmeans for withdrawing said inner-casing fronrsaid'outer casing.

4. A pump comprising a-.Inultistageimpeller unit, an inner casing defining the'workingpas .sageslior the several impellers of said impeller unit. an. outer casing spaced circumferentially from saidinner casing, aligned fluidinlets for said casings adjacent one end thereof, aligned fluid outlets for said casings communicating with. said space, said outer casing having an opening at the suction end thereof for assembly and disassembly of the inner casing relative to the outer casing, annular shoulders provided on said outer and inner casings between said open end and said out? lets, and means closing the'open end of said outer casing and bearing against said inner casing for clamping said shoulders into seating engagement. 5. A multistage centrifugal pump comprising at least a first, a second and a discharge stage, a shaft having an impeller element arranged thereonfor each of the said stages, an inner casing defining a working passage for each of said'impeller elements, saidfirst. and second stages being at opposite ends of said pump, an outer casing spaced circumferentially from said inner casing and having an open end, cover means rigidly securing said inner casing to said outer casing at said open end, a shaft opening extending through the oppositeend wall of said outer casin an axial extension ofsaid inner casing. slidably arranged within said shaft opening, a shaft packing means mounted in said opening, a inlet for said pump communicating .with said flrststage wherehysaid covermeans is subjected only, to inlet ,flui'd pressure, a passage in saidcasingfor conducting fluid discharged bysaidflrst stage to said. second stage whereby the shaft packing means atthe opposite end of "s'aidipump is subjected only 'to inlet fluid pressure plus first stage pressure, anda passage throu'gl'i saidfcasing for conducting, nuid'f 6m said second stage tolsaid discharge stage.

6. A centrifugal pump comprisi'ngan cute c ing having an open end, an inner casing "having enis l r mea am s wiil n WW meni es-Sec i hin r r es. was outer casingat said openend, a longitudinally e ten in y insr s recess i he e d i l f df wall of the said outer casing, an axial extension ofsaid inner casing slidably arranged. withinsaid.

i i s'a m i 'si ai lin m an 1 the space between said innerand outer cas; I

7.; ingfhavingan open end, an inner casing having an impeller arrangedtherein, cooperating inlets in saidicasings adjacent said open end, cooperatmgbutletslin said casings spaced axially from said inlets on the opposite side thereof with respect to said open end, a removable cover for said open end, said cover being subjected only to the suction pressure obtaining in said inlets,

means securing said inner casing to said cover,

a portion of said inner casing slidably arranged within the end wall of said outer casing at the opposite end with respect to said open .end for permitting relative longitudinal expansion movements therebetween. u

8. A centrifugal pump comprising an outer casing having an open end, an inner casing mountedin said outer casing and spaced circumferentially therefrom, a fluid inletlpassage through said casings adjacent said open end, a fluid outlet passage through said casings on the opposite side of said inlet passage with respect to said open end and communicating with said circumferem tial space, annular means between said casings adjacent said open end for sealing said circumferential space from said inlet passage, a cover for said open'end, an axial cylindrical opening in the opposite end wall of said outer casing, an axial cylindrical portion of said inner casing extending into said opening, an outwardly facing annular shoulder on said outer casing, sealing means seated at one end against said annular shoulder and engaging with the circumferential walls of said outer and innercasings, and means forming a part of said outer casing for compressins, said sealing means for sealingthe correspond I ing end of said circumferential space.

9. A centrifugal pump comprising an outer casing having an open end, an inner casing spaced circumferentially from said outer casing, outlets through said casings and communicating with said circumferential space, means closing said open end and securingthe corresponding end of said inner casing relative to said outer casing, an axial cylindrical opening in the opposite end wall of said outer casing, an axial cylindrical extension on said inner casing slidably arranged within said opening whereby the corresponding end of said inner casing is supported in said opposite end of said outer casing, sealing means arranged between said outer casing and said cylindrical extension of said inner casing for sealing the corresponding end of said circumferential space from a low pressure area in said inner casing while permitting relative movement of A' centrifu'ga1 pump comprising an outer, casf saidcasings, and, an annular compression member forl'maintainingj cbnstant compression of ",mean's'irrespecn e or the relative theltwo' casmg'f is.if

ugal pump comprising an outer ing,an impellerun t arranged therein, said inner I a cover forfsaidf open end, an end casingfbeingsecured to said covere'd 'meieby'relativeto said outercas "m-tnuarsp ce, inlet for said pump op lend; nu arjmeans arran ed d, teens-seamen said open end he: inlet: sa d'ioutlet mrsealme said.

al inf the'foppos'ite end wall of an axial cylindrical portion ,of

' tendingginto said opening,

ner casing portion, an

t nnular shoulder on said outer casing; and inansiorming'alpart of said-outer casing'for compressing said annular member between said annular shoulder and said means whereby said corresponding end of said circum- Ierential space is sealed but permitting axial expansion movements between said corresponding casing ends.

11-. Afoentrifrugal pump comprising an outer casing havingan open end, an inner casing having an impeller unit including a shaft arranged therein, said izmer-casing being circumferentially spaced from said outer casing, a cover for said open end, one end ofsaid inner casing being secured to said cover and held fixed thereby relative to said outercasing an inlet for said pump adjacent said open end, an outlet for said pump communicating with said circumferential space, annular means arranged between said casings between said inlet and said outlet for sealing said circumferential space from said-inlet, an axial 45 cylindrical opening'in the opposite end wall of said outer casing, an axial cylindrical portion of said inner casing extending into said opening, an annular compressible member arranged in said opening and engaging with the circumference-of said inner casing portion, and additional means closing saidopening about said shaft and defining a space between it and said compressible memher in communication with the interior of said inner casing.

12. A centrifugal pump comprising-an outer casing having an open end, an inner casing having an impeller unit includinga shaft arranged therein, said inner casing being cir-cumferentially spaced from said outer casing, a cover for said open end, an end of said inner casing being se-' cured to said cover and held'fixed thereby relative to said outer casing, an inlet for said pump adjacent said open end, a pair of diametrically opposed outlets for said inner casing communiin said outer casing communicating with said circumferential space, means arranged adjacent said open end between said casings between said inlet and said outlets'for sealing said circumferential space from said inlet, an axial cylindrical opening in the opposite end wall of said outer casing, an axial cylindrical portion of said inner casing extending into said opening, an outwardly facing annular shoulder on said outer casing, an annu- 76 lar compressible member arranged in'said openca's'ing having an" open end, an inner casing 'havcasingf 'circuinfei'ent ally spaced from said t al "space from said inlet, an axial r compress bleseaiingmember arranged r asing'and the circumferenceeating with said circumferential space, an'outlet' fining working passages jimeach or said impeller elements, the "discharge stage; impeuer element being" disposed interm'ediately of'saidplurality of ir'ri'peller elements, said 'discliargejstage and an adjacent intermediate? pr ssure stage impeller elements'hajving externallyjthr'eaded hub portions extending toward each, Iothen'j "an' internally threaded sleeve "cooperatively threaded upon said hub portions "fixedly, securing "said portions together in a predeterminedaili i ation; a Second sleeve concentrically surrounding said threaded se m a esv w en lag between said discharge stege'and said intenmediate pressure stage, said partition wall receiving'and retaining said second sleeve stationarily about said shaft;

14. A centrifugal pump comprising a plurality of stages, an outer casing, an inner casing spaced cir'cumferentially from said outer casing, animpeller unit arranged in said inner casing and having a plurality of axially spaced impeller elements thereof, said inner casing defining-working pas sages for said impeller elements, the discharge stage being disposed intermediately of said plurality of stages, an intermediate pressure stage adjacent said discharge stage having an impeller element with the inlet facing in the opposite direaction with respect to the inletof the discharge stage impeller element; salddischarge-stage and intermediate pressure stage impeller elements having externally threaded hub portions extending toward each other, an internally threaded sleeve cooperatively threaded upon said hub portions securing said portions together in a predetermined spaced axial relation.

' -ARTHUR. J. NELSON. 

